1. Field of the Invention:
The present invention relates to a method and means for laying a marine pipeline and more particularly, but not by way of limitations, to a reel upon which the pipe to be laid is wound and a straightening mechanism for straightening the pipe as it is unwound.
2. Prior Method:
Oil and gas marine pipelines are commonly laid on the bottom of the body of water. The most effective method for laying long links of pipeline in marine locations involves welding short sections of pipe together at a shore location and then coil them onto a barge mounted reel. The barge is them moved to the location at which the pipe is to be laid and the pipe is unwound from the reel. However, when winding the pipe onto the reel in the first place, the pipe metal yields in bending as it conforms to the reel surface. Therefore, the pipe must be yielded in the opposite direction or straightened as it is uncoiled from the reel for laying on the floor of the body of water. The straightening is accomplished by an arrangement of either two or three rollers which are configured to continuously stretch the pipe in bending in opposite directions within the plane of the coil thus effecting a continuous straightening of the pipe. The straightening rollers are typically located a distance from the reel so that the pipe will enter the rollers at a reasonably constant angle which is preferred to be normal to the roller axis and in a plane parallel to the axis of the rollers.
If the pipe is unwound from the bottom of the reel which has a horizontal axis, there is a tangent line from the bottom of the reel to a roller at the stern of the barge over which the pipe drops or leads into the body of the water. A first straightening or bending roller adjacent the reel deflects the pipe to a substantial distance above this tangent line. A second straightening roller is between the first straightening roller and the stern roller. This second straightening or reaction roller has a lower peripheral surface which is closer to the tangent line than is the surface of the pipe at the point of contact between the pipe and the surface of the first straightening roller. In this system static beam load theory is used to determine the various forces. The theoretical beam would be supportive at one end at the tangential point of the reel and at the other end by the support of the reaction roller. The downward force is at the first straightening roller caused by the pipe. This downward force in the beam theory is located a distance between the ends of the radiant beam which is determined by the position of the first straighening roller between the tangential point of the reel and the reaction roller.
When laying the pipeline, there is nearly always tension maintained on the pipe itself. This is obtained by having the pipe that is laid anchored to the sea floor and by applying the tension at the barge. The pipe goes over the stern of the barge and is shaped or held in a certain direction by use of a ramp, commonly referred to as a stinger.